Fouling of heat transfer surfaces due to coke formation is a significant problem in refinery equipment and pyrolysis furnaces used for the high temperature processing of hydrocarbon feedstocks.
In particular, ethylene manufacture involves the use of pyrolysis furnaces (also known as steam crackers or ethylene furnaces) to thermally crack various gaseous and liquid petroleum feedstocks to ethylene, propylene and other useful products.
A typical pyrolysis furnace has three building blocks: convection section, radiant section, and transfer line exchanger (TLE). Steam is generally injected into the pyrolysis furnace in addition to petroleum feedstocks. The convection section is a heat exchange device to recover exhaust heat and to preheat feed. Petroleum feedstocks and steam are fed into convection section coils, where the petroleum feedstocks and steam are mixed and preheated to desired temperatures ranging from 400 to 700° C. The hot mixture of the petroleum feedstocks and the steam (hereinafter “feed”) is then sent to the radiant section. The radiant section is the reactor where the petroleum feedstocks are thermally cracked at temperatures ranging from 700 to 1000° C. The radiant section reactor itself is Ni—Cr—Fe alloy tubes with diameters between 2 to 9 inches. The effluent exits the radiant section at a temperature from 750 to 870° C., and this effluent is immediately discharged to TLE. A TLE is a heat exchanger, and its function is to quickly quench the hot radiant section effluent to about 250° C.
The effluent from TLE is further cooled through oil and/or water quench towers, and then fractionated and purified in the downstream processes to desired products.
Ethylene and propylene are two of the major and the most desired of the products.
Carbonaceous material, known as coke, forms as the by-product of the cracking reactions in pyrolysis furnaces. Fouling of the radiant reactor coils and TLEs occurs due to the coke formation. The coke formation and fouling often becomes the major limitation in pyrolysis furnace operation. The coke formation and fouling decreases the effective cross-sectional area of the process feed flow, and thus increases the pressure drop across pyrolysis furnaces. The pressure buildup in the radiant reactor adversely affects product yield of desired products. Generally, a reduction in feed rate is necessary to compensate for the pressure buildup, resulting in a cut in production. Additionally, coke is a good thermal insulator, and thus the coke buildup inside of a radiant reactor requires a gradual increase in furnace firing to ensure enough heat transfer to maintain the cracking reactions at a desired conversion level. The fouling in TLE's also decreases the effective cross-sectional area of flow, which reduces heat transfer efficiency of the TLE's or causes pressure buildup. Depending on the coking and fouling rate, pyrolysis operation must be periodically shut down for coke removal.
The coke removal from pyrolysis furnaces is carried out using a mixture of steam and air of various steam/air ratios to burn out the coke in the pyrolysis furnaces (decoke). The coke removal from TLEs often requires both the decoke and a subsequent off-line mechanical cleaning. In addition to the periodic cleaning, crash shutdowns are sometimes required because of dangerous situations resulting from coke buildup in the pyrolysis furnaces.
The pyrolysis operation down time, capacity reduction, and ethylene yield deterioration lead to production loss. Coke formation and fouling also stresses pyrolysis operation and shortens pyrolysis furnace lifetime. Therefore, any process improvement or chemical treatment that could reduce coke formation and fouling would increase production and lower maintenance costs.
Coke inhibitors are chemical additives used to treat heat transfer surfaces to prevent coke formation and fouling. Organophosphorus compounds containing phosphorus-sulfur bonding, such as mono- or di-substituted thiophosphate esters, phosphorothioites, phosphorothioates and thiophosphonates, (hereinafter “phosphorus-sulfur compounds”) are known antifoulants to prevent coke formation and fouling on heat transfer surfaces of refinery and petrochemical plant equipment.
U.S. Pat. No. 3,647,677 discloses a method of using triethylthiophosphite, as crude oil additive to retard coke formation on refinery equipment. U.S. Pat. No. 4,024,048 discloses a method of treating hydrodesulfurization equipment with phosphate and phosphite mono- and di-thioester antifoulants. U.S. Pat. Nos. 4,024,049 discloses a method of treating the equipment in a crude oil system with thio-phosphate and phosphite mono- and di-esters to prevent fouling. U.S. Pat. No. 4,226,700 discloses a method of preventing fouling on refinery equipment using a combination comprising thiodipropionate and phosphate/phosphite diesters/thioesters. U.S. Pat. No. 4,542,253 discloses water soluble amine neutralized mono- and di-substituted thiophosphate esters for reducing fouling and corrosion in ethylene furnaces. Canadian patent No. 1,205,768 discloses morpholine-neutralized phosphate and thiophosphate esters as ethylene furnace anti-coking antifoulants. U.S. Pat. No. 5,354,450 discloses phosphorothioates for inhibiting coke formation in ethylene furnaces. U.S. Pat. No. 5,779,881 discloses phosphonate/thiophosphonate for inhibiting coke formation in ethylene furnaces.
In practice, the injection of the additive generally requires a pump, an injector and an injection line which connects the pump and the injector. An injector is essentially a piece of tubular pipe insertion into a pyrolysis furnace coil, and its function is to transport the additive into pyrolysis coils. The injector can be as simple as a piece of high alloy tubing or as sophisticated as an atomizer. The inlet end of the injector is located outside of the coil and connected to the injection line. The outlet end is located inside of the coil and the additive discharged to the process stream at the outlet end. Carrier gases are often used to facilitate the delivery of the additive and the distribution of the additive in process feed. As indicated above, there is no chemical treatment or preparation of the additive, except physical delivery, during the process of injecting the additive.